The invention relates to chemically stable universal glasses which can be melted without addition of boron-containing raw materials (boron-free universal glasses).
The term “universal glass” encompasses glasses which have very good chemical resistance and low thermal expansion and are suitable for wide industrial application and commercial use.
Known universal glasses include, inter alia, Duran® from Schott AG, Mainz, DE and Pyrex® from Corning Inc., USA. Borosilicate glasses having about 13% by weight of B2O3 in the synthesis have been known for a long time. Owing to the low thermal expansion, they have a high temperature change resistance (TCR) and are therefore stable to changes in temperature during use of the glasses.
Furthermore, these glasses are “neutral” in that they do not release any significant amounts of glass constituents into solutions present therein. They therefore also belong to the group of “neutral glasses” and can be used as primary packaging materials in the pharmaceuticals industry, in particular for injection solutions.
The measurements and classifications of the chemical constituents of industrial glasses are carried out according to standardized measurement methods as per table 1:
TABLE 1Classes of chemical resistance for industrial glassesResistance to . . .WaterAcidAlkali(H)(Ac)(Alk)StandardDIN ISO 719DIN 12116DIN ISO 695SolutionDist. H2O6N HCl (half-conc.)0.5M Na2CO3 +0.5M NaOHTemperature98° C.108° C.102° C.Time1 hour6 hours3 hoursUnitμg of Na2O/g ofmg/dm2mg/dm2glassClass 1up to 31up to 0.7up to 75Class 2up to 62up to 1.5up to 175Class 3up to 264up to 15above 175Class 4up to 620above 15Class 5above 620
Neutral glasses are within the first hydrolytic class and first acid class and at least in the second alkali class; they are therefore referred to here as “1-1-2 glasses”.
Boron-free glasses naturally cannot meet the requirements of the standard DIN ISO 3585 (“Borosilicate glass 3.3—Properties”) which have to be met by a laboratory glass, i.e. chemically and thermally stable glasses for apparatus construction, since the glasses in question here are explicitly borosilicate glasses and the property values in their totality set down in the standard can also only be met by borosilicate glasses. However, boron-free glasses are in principle also suitable for use as laboratory glass since they can have not only the very good chemical resistances (1-1-2 glass) but also a very low thermal expansion.
Although boron oxide is present in SCHOTT Duran® 8412 in an amount of only about 13%, the boron raw materials incur the predominant part of the total raw materials costs. The raw materials situation for borosilicate glasses without sodium oxide, e.g. alkali metal-free glasses for LCD display screens, is even more unfavorable because in this case the much more expensive raw material boric acid, which firstly has to be obtained industrially from borax, has to be used. The costs for the glass component B2O3 from the raw material boron oxide are about seven times as high as the costs for B2O3 from the raw material disodium tetraborate pentahydrate.
The EU (European Union) has recently classified boric acid, diboron trioxide, anhydrous disodium tetraborate, disodium tetraborate decahydrate and disodium tetraborate pentahydrate as having reproductive toxicity. As a consequence, particular boundary conditions have to be adhered to and particular precautionary measures have to be taken during production using such raw materials.
Owing to the relatively high costs of boron-containing raw materials, the foreseeable shortage of suitable qualities and the current discussion on new toxicity classifications for boron compounds, boron-free glasses are of interest as alternatives to borosilicate glasses.
For use as substrate glass, e.g. as solar glass, touch panel glass, the good chemical resistance of boron-free glasses is advantageous since these substrate glasses are subjected in most production processes to cleaning using aqueous solutions and acids, after which the glass surface must not display any changes. Good chemical resistance is likewise advantageous for good weathering resistance.
Apart from very good chemical resistance, further requirements have to be met by universal glasses or neutral glasses.
Thus, the glass has to be able to be produced in conventional melting apparatuses, i.e. the viscosity of the melt must not be too high—the processing temperature (temperature at which the viscosity is 104 dPas, also referred to as VA or T4) should in no event exceed a maximum value of 1320° C. T4 should be as low as possible in the interests of energy-saving production.
A further parameter for producibility is sufficient devitrification stability, i.e. the tendency to form crystals from the melt during production should be very low.
Many boron oxide-free glasses are described as “chemically stable” in the literature without information being given on material removal values or the like, or the information on chemical stability cannot simply be carried over to the standards in table 1.
Although a series of boron-free glasses are known from the prior art, these are essentially unsuitable as universal glasses according to the present definition.
The document JP 10-045422 discloses a glass having the composition 66-72 mol % of SiO2, from 10 to 14 mol % of Al2O3, from 0 to 1.5 mol % of B2O3, from 0 to 10 mol % of MgO, from 0 to 10 mol % of CaO, from 0 to 10 mol % of SrO and from 0 to <1 mol % of BaO, which has a thermal expansion a in the range from 20° C. to 300° C. of ≦4 ppm/K and a processing temperature T4 above 1300° C. The processing temperature T4 is too high for economical production. The acid resistance is also poor.